A quantum physicist's view of hydrogen bonding

Hydrogen bonding plays a central and diverse role in chemistry and biology. It is key to the unique properties of water, the double helix structure of DNA, and the unique folding of proteins. Yet it is arguably the most poorly understood form of chemical bonding. Indeed the International Union of Pure and Applied Chemistry (IUPAC) recently gave a new definition of hydrogen bonding.

As a physicist I recently investigated hydrogen bonding with three goals:
i. to use the simplest possible model
ii. to describe a wide range of phenomena
iii. to elucidate the role of quantum physics in hydrogen bonding.

I consider a model which describes hydrogen bonding and proton transfer between two molecules due to the quantum mechanical interaction between the orbitals of the H-atom and of the donor (D) and acceptor (A) atoms in the molecules [1].

The model is based on a effective Hamiltonian which acts on two diabatic states and has a simple chemically motivated form for its matrix elements.

The model gives insight into the "H-bond puzzle" [2], describes different classes of bonds (weak, low-barrier, and strong), and gives a quantitative description of empirical correlations between the donor-acceptor distance and binding energies, D-H bond lengths, the softening (hardening) of D-H stretch (bend) vibrational frequencies.A key testable prediction of the model is the UV photo-dissociation of symmetric H-bonded complexes via an excited electronic state with an exalted vibrational frequency.

[1] R.H. McKenzie, Chemical Physics Letters 535, 196 (2012).
[2] G. Gilli and P. Gilli, The Nature of the Hydrogen Bond (Oxford UP, 2009).